A number of processes heretofore have been employed to grow crystals of various materials from a melt. One such process is the Bridgman-Stockbarger process wherein a seed crystal is positioned at the base of the crucible into which melt stock is loaded. The crucible is heated to form a melt that varies in temperature from the seed temperature to higher temperatures upward in the melt. The crucible is then lowered slowly from a high temperature melt zone to a lower temperature cooling zone to induce crystal growth from the seed crystal at the base of the crucible to the top of the melt.
Another crystal growth technique is generally referred to as vertical gradient freeze process. This process maintains the crucible stationary while otherwise inducing vertical crystal growth in the crucible. For example, overall heating power may be lowered so as to move the melting point isotherm upward through the crucible. Another procedure involves use of a heat exchanger to extract heat from the base of the crucible.
The ability to grow high quality crystals with acceptable yields depends in large part on the ability to monitor and control the solid-liquid interface position. This is not an easy task given the environment in which the crystals are grown. One method to locate the solid-liquid interface is to physically contact the crystal at the interface with a probe. This method, however, may cause growth defects at the interface and may leave debris particles from the probe and the crucible lid through which the probe passes, thus contaminating the melt.
Another method that has been proposed uses ultrasonics. The interface position is determined using the difference in ultrasonic velocity between solid and liquid phases. This method, however, suffers from low resolution which compromises the accuracy of crystal growth.
A further method that has been proposed uses X-ray radioscopy. This method is based on the principle that X-ray absorption for a given material is a function of density gradient between solid and liquid phases. This method, however, requires furnace shielding, is difficult to practice in an industrial environment, and suffers from low accuracy due to hot zone absorption.